1. Field of the Invention
The present invention relates to a delay line structure, and more particularly to a flat spiral delay line structure with grounding guard traces.
2. Description of the Prior Art
Signal synchronizing is always a concern when it comes to the high speed digital signal. Usually the requirement of synchronizing signals is met by increasing delay time with a delay line.
To save space, the delay line is usually bent. There are various delay lines, among which a flat spiral delay line is very common. Referring to FIG. 1, FIG. 1 illustrates a flat spiral delay line structure in prior art. A flat spiral delay line 11 is bent inward clockwise, then bent outward counterclockwise and is arranged on a substrate 100.
However, bending the delay line forms several coupling line segments in the delay line itself, also generates crosstalk noise disturbance and further affects a signal waveform received—which leads to misinterpretation on a voltage level of a digital signal. As a result, in the prior art, a guard trace with both ends connecting to the ground is utilized for decreasing the crosstalk noise. Referring to FIG. 2, FIG. 2 is a schematic view showing a guard trace with both ends connecting to the ground being applied to a flat spiral delay line in prior art. The flat spiral delay line 11 is bent inward clockwise, then bent outward counterclockwise and is arranged on a substrate 100, and therefore a spiral coupling area H1 with an opening and a spiral coupling area H2 with an opening are formed. A guard trace 12 and a guard trace 13 are inserted in the coupling area H1 and coupling area H2 separately. Moreover, two ends of the guard trace 12 are electrically connected to a grounding layer of the substrate 100 by a via 121 and via 122 while two ends of the guard trace 13 are electrically connected to the grounding layer of the substrate 100 by a via 131 and via 132.
Although the guard traces 12 and 13 can efficiently decrease the crosstalk noise of the flat spiral delay line 11, the coupling area H1 and H2 must be large enough for the vias 122 and 132 to be disposed inside the flat spiral delay line 11, which is difficult to process. That is why, downsizing a structure for the flat spiral delay line 11 is still a difficulty. Meanwhile, since the guard trace 12 and 13 with two ends connecting to the ground can merely be utilized in a structure of microstrip line, the utilization is still limited.